The invention relates to methods of fabricating parts out of composite material comprising a fiber preform and a refractory ceramic matrix.
The present invention relates to a method of fabricating a part out of thermostructural ceramic matrix composite (CMC) material, i.e. material comprising fiber reinforcement formed from fibers made of refractory ceramic material with the pores being filled in with a matrix, likewise made of refractory ceramic material, and the invention relates in particular to a method of fabricating a part of the oxide/oxide type.
Parts made out of oxide/oxide composite material are generally prepared by draping a plurality of fiber plies in a mold, the plies being made from refractory oxide fibers, each ply previously being impregnated with a slip filled with refractory oxide particles. The set of plies as arranged in this way is then compacted using a countermold or a vacuum sheet. When a vacuum sheet is used, the preimpregnated plies may for example be passed to an autoclave (a method of the preimpregnated organic matrix composite (OMC) type). The filled preform as obtained in this way is then subjected to sintering in order to form a refractory oxide matrix in the preform and obtain a part made of oxide/oxide composite material. That technique can also be used to make other parts out of ceramic matrix composite (CMC) material. Under such circumstances, the fiber plies may be made from fibers of silicon carbide (SiC), or of carbon (C), and they may be impregnated with a slip filled with particles of carbide (e.g. of SiC), of boride (e.g. of TiB2), of nitride (e.g. of Si3N4), or of oxide, e.g. alumina or zirconia.
Nevertheless, that type of method of preparation can be used only to make ceramic matrix composite material parts that are of small thickness and with fiber reinforcement that is two-dimensional (2D). The mechanical characteristics of such composite materials are very different depending on the inherent directions of the reinforcing structure. More precisely, those materials present little resistance to delamination and to forces not in their plane.
Fiber textures obtained by three-dimensional weaving between continuous warp and weft yarns enables the mechanical strength of the material to be increased, and in particular enables its ability to withstand delamination to be increased. Under such circumstances, and also for thick 2D fiber textures, it is possible to cause a filled suspension to penetrate into the fiber texture, which may be of thickness that reaches several tens of millimeters, depending on the intended applications, only by means of methods that make use of a pressure gradient, such as infusion type methods, injection molding type methods known as resin transfer molding (RTM), or methods involving suction of submicron powder known as advanced powder solutions (APS).
Nevertheless, in the context of making a part out of ceramic matrix material, those methods present certain drawbacks.
Specifically, a fiber texture of complex shape and of considerable thickness cannot be impregnated by an infusion type method, since that type of method does not enable a sufficient pressure gradient to be achieved for obtaining good impregnation of the entire texture. The APS type method does not enable the volume fraction of the matrix inserted into the preform to be finely controlled, nor does it make it possible to control surface states finely.
Although the RTM method can be used for impregnating a fiber texture with a filled slip, it nevertheless requires a step to be performed of eliminating (discharging and/or evaporating) the liquid medium of the slip so as to leave only the solid fillers in the preform prior to sintering. That additional step lengthens the time required to perform the method.
Furthermore, the step of eliminating the liquid medium can lead to a loss of particles and/or to a change in the way the particles are distributed within the preform, and thus to the appearance of macropores in the final material as a result of the matrix missing in certain locations.
Account should also be taken of the fact that in RTM methods using a step of injecting the slip, it can be necessary to limit the volume fraction of refractory ceramic particles present in the slip in order to conserve viscosity that is low enough to enable the slip to be injected uniformly. This constraint can make it necessary to repeat the slip injection step, and consequently the step of eliminating the liquid medium, so as to end up with the desired volume fraction for the matrix. The limit on the volume content of refractory ceramic particles in the slip can thus complicate the fabrication method.
Consequently, there exists a need to simplify methods of fabricating preforms filled with refractory particles for the purpose of forming composite material parts presenting a desired matrix volume fraction.
There also exists a need to have methods that are fast and reliable for fabricating composite material parts that present the desired properties from a fiber texture that is thick and/or of complex shape.